In general, electrostatic printing processes involve creating an image on a photoconductive surface, applying an ink having charged particles to the photoconductive surface, such that they selectively bind to the image, and then transferring the charged particles in the form of the image to a print substrate.
Electrostatic printing systems will now be described in a little more detail. The photoconductive surface is typically on a cylinder and is often termed a photo imaging plate (PIP). The photoconductive surface is selectively charged with a latent electrostatic image having image and background areas with different potentials. For example, an electrostatic ink composition comprising charged toner particles in a liquid carrier can be brought into contact with the selectively charged photoconductive surface. The charged toner particles adhere to the image areas of the latent image while the background areas remain clean. The image is then transferred to a print substrate (e.g. paper) directly or, more commonly, by being first transferred to an intermediate transfer member, which can be a soft swelling blanket, and then to the print substrate. Variations of this method utilize different ways for forming the electrostatic latent image on a photoreceptor or on a dielectric material.
In some systems, the electrostatic ink composition is applied to the PIP by pumping the ink between a stationary electrode and the PIP. Such a system is illustrated in WO 2005/054959, which is incorporated herein by reference in its entirety. Other electrostatic printing systems include a binary ink development (BID) unit. In such a system, the ink is applied to the photoconductive surface by a developer roller. Often, a different developer roller is used for each different color ink (e.g. cyan, magenta, yellow and black). Ink is applied to the developer roller by passing an electrostatic ink composition between a stationary charged electrode and the developer roller. Ideally, the charged toner particles should form a uniform layer on the development roller. The developer roller rotates, such that the charged particles contact the PIP electrically. Such a system is illustrated in U.S. Pat. Nos. 5,436,706, 5,610,694 and 5,737,666, all of which are incorporated herein by reference in their entirety.
Typically, an electrostatic ink composition comprises a thermoplastic resin as the basis for the toner particles, and a non-polar liquid as a liquid carrier in which the toner particles are dispersed. Generally, the toner particles contain a colorant such as a pigment. A charge director, also called charge control agent or imaging agent, is also typically added to the dispersion to induce charge on the particles. The content of the liquid carrier in the electrostatic ink is typically 75 to 80 weight % when shipped to printers. The ink may be further diluted to about 98 weight % liquid content before it is used in a printing press.
The liquid carrier, particularly hydrocarbon liquid carriers, play an important role in achieving high offset-like print quality with the electrostatic printing presses. The liquid carrier is believed to swell, to some extent, the resin particles within the ink, and aid transfer to the PIP, then the intermediate transfer member, and finally to the print substrate, e.g. paper. The ink on the print substrates may contain less than 5 weight % oil, with the rest of the oil having been excluded from the ink in a number of previous stages in the printing process.
There is a belief in the art that concentrating liquid electrostatic inks to a solids content of 30-35 wt % or higher irreversibly changes the particle structures in the ink, degrading the print quality and making the ink unsuitable for use on the electrostatic printing presses.
WO 2011/001199, which is incorporated herein by reference in its entirety, describes ways of concentrating electrostatic inks that avoid degrading the ink quality. This document describes a preferred method of concentrating an ink using centrifugal force. Example 1 of this document describes using a centrifugal separator to concentrate an ink containing about 22 weight percent solids to an clumps of agglomerated particles that had a solids content of 47 weight percent. In Example 2, the product from Example 1 was allowed to stand in air for 25 to 50 hours to allow the liquid carrier to evaporate. The solids content of the product after this period was 60 wt %.
While the method described in WO2011/001199 goes some way to providing a way of concentrating inks without adversely affecting their quality, further improvements could be made.